1. Field of the Invention
This invention relates generally to a powertrain for a hybrid electric vehicle (HEV) having an engine, an electric machine and a multiple-speed, powershift transmission. In particular, the invention pertains to using the powertrain to prevent rollback of the vehicle on an incline.
2. Description of the Prior Art
A powershift transmission is an example of a power transmission for a motor vehicle in which there is no torque-converter. A powershift transmission is a geared mechanism producing multiple gear ratios in forward drive and reverse drive and having two input clutches, which connect a power source, such as an engine or electric motor, to two transmission shafts. A powershift transmission transmits power alternately to the two shafts using synchronized clutch-to-clutch shifts.
A powershift transmission incorporates gearing arranged in a dual layshaft configuration between the transmission input and its output. One input clutch transmits torque between the input and a first layshaft associated with even-numbered gears; the other input clutch transmits torque between the transmission input and a second layshaft associated with odd-numbered gears. The transmission produces gear ratio changes by alternately engaging a first input clutch and running in a current gear, disengaging the second input clutch, preparing a power path in the transmission for operation in the target gear, disengaging the first clutch, engaging the second clutch and preparing another power path in the transmission for operation in the next gear.
In a motor vehicle whose powertrain includes a powershift transmission the vehicle has a tendency to rollback when the driver depresses the accelerator pedal (called a tip-in) following a hill-hold condition, in which the vehicle is held stationary on an incline with or without use of the wheel brakes. The rollback condition is caused by delay in starting the engine in the case where engine is shut down, delay in engine torque due to charging the intake manifold and cylinders with a combustible fuel/air mixture, and delay in producing input clutch torque capacity. These and other delays cause delay in producing wheel torque sufficient to hold the vehicle against rollback on an incline.
When a HEV comes to a stop and the battery's state of charge (SOC) is sufficient and other conditions are met, the engine is shut off. The engine could also be shut off during a hill-hold condition since the vehicle is stopped. Hill holding a HEV occurs with the driver holding the vehicle by applying the wheel brakes, or, if the wheel brakes are released, an electric machine can provide hill-holding wheel torque.
Rollback prevention is required when the vehicle operator then depresses the accelerator pedal (called a tip-in) and expects to ascend the hill. If the engine is to provide torque to the wheels to launch the vehicle, a delay occurs before wheel torque increases sufficiently due to the delays in engine starting, manifold filling and input clutch activation. Rollback can also occur when a vehicle is ascending a hill and the wheel torque does not meet the increased road load due to increasing grade.
There is a need in the industry for a technique that eliminates unintended rollback of the vehicle when (1) the vehicle operator tips-in to accelerate the vehicle on an uphill grade from a stationary vehicle condition while the engine is shutdown; (2) the vehicle operator tips-in to accelerate the vehicle on an uphill grade from a stationary vehicle condition while the engine is running; and (3) when the ERAD is the only available torque source while the engine is shutdown and the vehicle is ascending a hill and the current wheel torque capacity does not meet the increased road load.